This invention relates to electrical circuits which measure the root-mean-square (RMS) value of electrical signals.
The RMS value of an electrical signal is an important signal parameter because it represents the signal's power.
One type of apparatus for measuring a signal's RMS value which is well known in the prior art measures the heat dissipated by the signal in a resistive load, using some sort of temperature transducer. However, such apparatus have the disadvantages of long settling time for the measurements and limited dynamic range.
The most common type of prior art apparatus for measuring RMS values uses electrical circuits to perform a sequence of operations on the signal which is equivalent to the mathematical expression defining the RMS value. For example, the mathematical definition of RMS requires squaring the instantaneous value of a signal, then averaging this over time, then taking the square root of this average value.
In the prior art, the practically universal means for performing the squaring and square-rooting operations is by using the logarithmic voltage-current characteristic of semiconductor diodes and transistors. This technique relies on the principle that the product of two numbers may be obtained by computing the logarithm of each of the two numbers, summing the two results, and then taking the anti-logarithm of the sum. This technique is popular because logarithmic amplifiers and summers are much easier to design than multipliers.
The most common type of prior art RMS measuring circuit includes three separate sub-circuits in cascade which perform the squaring, averaging and square-rooting operations, respectively. One disadvantage of such circuits is that their means for summing the logarithms of signals depends on precision trimming of resistors or matching of logarithmic elements. For example, a prior art RMS circuit may include a sub-circuit for squaring a quantity; i.e., for raising that quantity to a power of two. However, unless certain critical components are precisely matched or trimmed, the sub-circuit's output will actually equal the quantity raised to a power not quite equal to two. The resulting errors, because of their nonlinearity, are almost impossible to correct.
Another disadvantage of such prior art circuits is their complexity; i.e., their large number of components. One obvious consequence of using a large number of components is high cost. Another is that, when fabricated on an integrated circuit chip, the circuit occupies a large area on the chip, thereby decreasing the yield of the fabrication process or preventing the inclusion of additional circuits on the chip to perform related functions. A further disadvantage of using a large number of components is that this usually restricts the frequency response of the circuit.
An RMS measuring circuit that overcomes most of the disadvantages of the foregoing type of circuit is disclosed in U.S. Pat. No. 3,743,949 to Engel et al. The disclosed circuit uses a combination of four bipolar transistors and a capacitor in place of the aforementioned separate sub-circuits for squaring, averaging and square-rooting. The disclosed circuit requires no calibration adjustments to minimize nonlinearities. It also has the advantage of requiring far fewer components than the previously described type of circuit.
Although it has the important advantages just described, the RMS measuring circuit disclosed by Engel et al. has various inherent shortcomings. One shortcoming is that the RMS measurement suffers nonlinear errors because a time-varying proportion of the input is diverted from the logarithm-computing transistors to the base of one of the antilog-computing transistors. Another shortcoming is that the disclosed means for maintaining a substantially constant voltage across the antilog-computing part of the circuit comprises a zener diode which undesirably increases the current drain from the power supply. Such current drain can significantly reduce battery life in a battery-powered portable instrument. A further shortcoming of the circuit disclosed by Engel is that the high frequency response is limited by the interelectrode capacitance of the transistors.
Another shortcoming is that the disclosed circuit does not provide measurement results in logarithmic units. Logarithmic RMS measurements, typically in units of "decibels," are commonly preferred by engineers in the communications and acoustics fields. A further shortcoming of the disclosed circuit is that the output signal it provides has an alternating component that must be filtered out to obtain the desired RMS measurement result.
Various embodiments of the pesent invention overcome one or more of the above disadvantages.